1. Field of the Invention
This invention pertains to telecommunications, and particularly concentration of user traffic in a radio access network.
2. Related Art and other Considerations
In a typical cellular radio system, wireless user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network. Alternatively, the wireless user equipment units can be fixed wireless devices, e.g., fixed cellular devices/terminals which are part of a wireless local loop or the like.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks. The core network has two service domains, with an RNC having an interface to both of these domains.
One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UMTS is a third generation system which in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a radio access network providing wideband code division multiple access (WCDMA) to user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM-based radio access network technologies.
The transmission infrastructure of cellular mobile access networks is expensive, mainly because it carries the traffic of a large number of bases stations. In order to reduce transmission costs, traffic concentrator nodes are placed in the access networks. These nodes are able to aggregate the traffic of base stations over large links, such that a significant statistical multiplexing gain can be achieved on those large links.
If a concentrator node is introduced in the network, the cost of the concentrator node should be smaller than the cost saving that results from the traffic concentration. Also, introduction of the concentrator node should not deteriorate the end-to-end performance.
Asynchronous Transfer Mode (ATM) is becoming increasingly used in communication networks. ATM is a packet-oriented transfer mode which uses asynchronous time division multiplexing techniques. Packets are called cells and have a fixed size.
As illustrated in FIG. 1, an ATM cell consists of 53 octets, five of which form a header and forty eight of which constitute a “payload” or information portion of the cell. The header of the ATM cell includes two quantities which are used to identify a connection in an ATM network over which the cell is to travel, particularly the VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier). In general, modem communication networks can cross connect traffic flows to form logical end-to-end connections between all origin-destination pairs and thus create fully meshed logical networks. Such logical connections are known as virtual path (VP) connections (VPC). The virtual channel (VC) is one specific connection on the respective virtual path.
Between termination points of an ATM network a plurality of nodes are typically situated, such as switching nodes having ports which are connected together by physical transmission paths or links. The switching nodes each typically have several functional parts, a primary of which is a switch core. The switch core essentially functions like a cross-connect between ports of the switch. Paths internal to the switch core are selectively controlled so that particular ports of the switch are connected together to allow a cell ultimately to travel from an ingress side of the switch to an egress side of the switch. The switch changes the identifiers of the ATM cells (VPI and VCI) and routes the cells towards the appropriate physical interface.
Various aspects of ATM-based telecommunications are described in the following: U.S. patent application Ser. No. 09/188,101 [PCT/SE98/02325] and Ser. No. 09/188,265 [PCT/SE98/02326] entitled “Asynchronous Transfer Mode Switch”; U.S. patent application Ser. No. 09/188,102 [PCT/SE98/02249] entitled “Asynchronous Transfer Mode System”; U.S. patent application Ser. No. 09/188,102, entitled “Asynchronous Transfer Mode System Handling Differing AAL Protocols”; U.S. patent application Ser. No. 09/188,097, entitled “Centralized Queuing for ATM Node”; U.S. patent application Ser. No. 09/188,340, entitled “Cell Handling Unit and Method for ATM Node”; U.S. patent application Ser. No. 09/188,347, entitled “ATM Time-Stamped Queuing”; U.S. patent application Ser. No. 09/188,344, entitled “Coordinated Cell Discharge From ATM Queue”; U.S. patent application Ser. No. 09/188,096, entitled “Combined Header Parameter Table for ATM Node”; U.S. patent application Ser. No. 09/134,358, entitled “Cell Selection for ATM Switch Having Redundant Switch Planes”; U.S. patent application Ser. No. 09/213,897, entitled “Internal Routing Through Multi-Staged ATM Node”; U.S. patent application Ser. No. 08/893,507, entitled “Augmentation of ATM Cell With Buffering Data”; U.S. patent application Ser. No. 08/893,677, entitled “Buffering of Point-to-Point and/or Point-to-Multipoint ATM Cells”; and U.S. patent application Ser. No. 08/893,479, entitled “VP/VC Look-Up Function”, all of which are incorporated herein by reference in their entirety.
A protocol reference model has been developed for illustrating layering of ATM. The protocol reference model layers include (from lower to higher layers) a physical layer (including both a physical medium sublayer and a transmission convergence sublayer), an ATM layer, and an ATM adaptation layer (AAL), and higher layers. The basic purpose of the AAL layer is to isolate the higher layers from specific characteristics of the ATM layer by mapping the higher-layer protocol data units (PDU) into the information field of the ATM cell and vise versa. There are several differing AAL types or categories, including AAL0, AAL1, AAL2, AAL3/4, and AAL5.
AAL2 is a standard defined by ITU recommendation I.363.2. An AAL2 packet is shown in FIG. 2 as comprising a three octet packet header, as well as a packet payload. The AAL2 packet header includes an eight bit channel identifier (CID), a six bit length indicator (LI), a five bit User-to-User indicator (UUI), and five bits of header error control (HEC). The AAL2 packet payload, which carries user data, can vary from one to forty-five octets.
FIG. 3 shows how plural AAL2 packets can be inserted into a standard ATM cell. In particular, FIG. 3 shows a first ATM cell 201 and a second ATM cell 202. Each ATM cell 20 has a header 22 (e.g., cell 201 has header 221 and cell 202 has header 222). The payload of the ATM cells 20 begin with a start field 24 (e.g., cell 201 has start field 241 and cell 202 has start field 242). After each start field 24, the ATM cell payload contains AAL2 packets. For example, the payload of ATM cell 201 contains AAL2 packets 261 and 262 in their entirety, as well as a portion of AAL2 packet 263. The payload of cell 202 contains the rest of AAL2 packet 263, and AAL2 packets 264 and 265 in their entirety. In addition, the payload of cell 202 has padding 28. The start field facilitates one AAL2 packet bridging two ATM cells.
Thus, in ATM networks, cells are transported along predefined paths using the VPI/VCI (virtual path and virtual channel identifier) fields in the ATM header. For AAL2, each specific AAL2 connection within an ATM VC is identified by the CID (connection identifier) field in the AAL2 header. A CU timer (TCU) determines how long the multiplexer should wait for arriving AAL2 packets before transmitting a partly filled ATM cell. Therefore, multiplexing efficiency also depends on the value of TCU. In the case of highly utilized links, this dependency can be neglected.
In ATM/AAL2-based UMTS access networks (UTRANs), traffic concentration can be implemented by various combinations of different ATM and AAL2 features. The major issues involved are (1) the type of ATM Virtual Channels (VCs) to be used, i.e., CBR (Constant Bit Rate), VBR (Variable Bit Rate), or UBR (Unspecified Bit Rate), and (2) whether AAL2 switching or only ATM VC switching is to be used. Usually CBR Virtual Paths (VPs) are used. Four switching alternatives—alternative A through alternative D are described below with reference to FIGS. 4A-4D, respectively.
Alternative A (illustrated in FIG. 4A) uses ATM VC switches and CBR/rt-VBR VCs for traffic aggregation. Alternative A also utilizes end-to-end VCs, with VC capacities being defined such that the AAL2 Connection ID-limit (a maximum of 248 AAL2 connections can be multiplexed in a VC) is taken into account. If the VCs are too large, the CID limit may preclude full use of the VC capacity (having many narrow-band connections). On the other hand, if the VCs are too small, packet-level statistical gains are decreased and also granularity problems arise.
Alternative B (illustrated in FIG. 4B) uses AAL2 switches and CBR/rt-VBR VCs for traffic aggregation. Alternative B is similar to Alternative A, but there are no end-to-end VCs. As a result, in a concentration node AAL2 connections are switched from one VC to another.
Alternative C (illustrated in FIG. 4C) uses AAL2 switches and UBR VCs (but CBR VPs) for traffic aggregation. Alternative C is similar to Alternative B, but (in Alternative C) UBR VCs are used. Since there is no bandwidth assigned to UBR VCs, in FIG. 4C, 1 the ‘pipe’ of the VCs is not depicted. In the concentration nodes AAL2 connection admission control (CAC) over the VP resources can easily be done, because there are always AAL2 switches there. Alternative C is expected to have the best performance from statistical multiplexing point of view.
The use of VBR is problematic for several reasons. A first reason is that resource allocation for VBR is very complex in general. A second reason is that in UTRAN the AAL2 traffic descriptors do not contain enough information for VBR resource allocation. Therefore the use of CBR or UBR (in CBR VPs) is more straightforward.
Alternative A is simple, however, significant statistical multiplexing gains can not be achieved. With Alternative B and Alternative C the significant gains can be achieved, but the concentrator node (AAL2 switch) may be expensive, because AAL2 de-multiplexing and multiplexing needs to be done in the AAL2 switches as illustrated in FIG. 5. Furthermore, AAL2 switching introduces delays, which can be avoided if only VC switching is used. From the node implementation point of view, large AAL2 switches may be difficult to realize because, e.g., a large amount of processing capacity is needed.
Alternative D (illustrated in FIG. 4D) uses ATM VC switches and UBR VCs (but CBR VPs) for traffic aggregation. The CID limit is not a problem for alternative D, because there is no bandwidth associated with the UBR VCs. Using alternative D, AAL2 connection admission control (CAC) needs to be done over the CBR VP resource. A benefit of using alternative D is that AAL2 switching is avoided, but the same statistical multiplexing gain (as with AAL2 switching) can be achieved. A disadvantage with alternative D is that since only VC switching is done, the number of VCs used for transmitting AAL2 traffic is not reduced.
Alternative D is described, to some extent, in the following: (1) H. Saito, “Effectiveness of UBR VC Approach in AAL2 Networks and Its Application to IMT-2000”, IEICE Transactions on Communications, Vol. E83-B, No. 11, 2000; and (2) H. Saito, “Performance Evaluation of AAL2 Switch Networks”, IEICE Transactions on Communications, Vol. E82-B, No. 9, 1999.
A basic problem with Alternative D is that the AAL2 standard assumes that AAL2 endpoints can reside at places where the ATM VCs are terminated. Alternative D does not satisfy this assumption. Rather, Alternative D can only be implemented in an AAL2 endpoint which (1) has information about all the AAL2 connection requests and (2) knows the topology and the configuration of the network. In other words, the alternative D can only be implemented at a point which can do admission control for essentially the whole network. Such a location can be a radio network controller (RNC) node if the network has a tree topology. Therefore, it is not good practice, and extremely difficult to arrange, a concentrator point (comprising an ATM switch) configured to use alternative D while bandwidth management functions of the virtual channels are located elsewhere (at an RNC). If the network does not have a tree topology, alternative D is even more problematic. Thus, it is perplexing how to process AAL2 signaling messages in alternative D while performing AAL2 CAC in such a manner that the ATM infrastructure is not touched.
What is needed, therefore, and an object of the invention, is an efficient traffic concentrator for a telecommunications network such as a radio access network, for example.